This section deals with the methods of measurement on a digital radio signal processing sub-system. The digital signal processing sub-system is, in general, able to perform the following principal functions: — multiplexing of two or more bit streams (see IEC 835-2-9, "Service Channels", figure 1); — transmission quality evaluation for switching purposes and/or alarm indicating signal (AIS) insertion; - coding and multiplexing of digital service channels, (see note); — scrambling and descrambling; — series-to-parallel and parallel-to-series conversion. System configurations differ, so some of the above functions may be missing, in which case only the measurements relating to those functions present should be taken into consideration. In digital radio-relay systems, many system configurations exist where non-hierarchical bit rates are used. In such cases, multiplex equipment is used in order to transmit more than one hierarchical bit stream and/or additional service channels (e.g. 34 Mbit/s and 140 Mbit/s with bit insertion for parity bits and digital service channels, 2 x 34 Mbit/s, etc). For low bit rate systems, e.g. < 2 Mbit/s it may not be practicable to perform tests at very low BERs because of the excessively long measurement times involved. Additionally, in many cases parity bits are transmitted for quality control and switching purposes. However, as far as possible, the system should be tested at defined inte rface points such as those considered by the CCITT Recommendation G.703. NOTE - Digital service channels are dealt with in IEC 835-2-9.